Pyrophosphate copper strike zincating solution



United States Patent Conn. No Drawing. Filed Feb. 21, 1964, Ser. No. 346,392

34 Claims. (Cl. 117-50) The present invention relates to improved immersion zincate solutions and compositions designed for the creation of such solutions, for the preparation of aluminum and aluminum alloys for plating with pyrophosphate copper.

In a co-pending application Ser. No. 60,795, filed Oct. 6, 1960, now Patent No. 3,216,835, issued Nov. 9, 1965, there is described a process for preparing aluminum and aluminum alloy surfaces for plating which comprises the treatment of such surfaces with dilute zincate solutions. Such procedures are generally free of metallic salts or other activators such as salts of copper or iron. After the zincating step in the preparation of aluminum and its alloys for plating, it is customary to copper strike the part being plated. Most copper strike solutions include cyanides although other types are less commonly used, such as pyrophosphate.

Commercial experience has shown that zincating baths of the type described in US. Patent 3,216,835 cannot be used satisfactorily for preparing aluminum and aluminum alloys for plating, when a pyrophosphate copper strike is employed. The reason for this is the relatively high activity of the copper ion in the pyrophosphate bath. This results in an immersion deposit of copper on zinc whereby the poor quality of the resulting immersion copper deposit causes blisters to appear on the part being plated after the final plated coating has been applied.

The present invention relates to a modified formula of Patent 3,216,835, referred to, to permit the plating of aluminum and its alloys With a pyrophosphate copper strike, while retaining the advantages of using a dilute zincate formula.

We have discovered that the addition of a small amount of one or more copper salts to the zincating formulations disclosed and claimed in US. Patent 3,216,835, previously referred to herein, will reduce the activity of the zinc immersion deposit sufficiently to prevent an immersion copper deposit from forming thereon in a pyrophosphate copper strike. This allows the use of dilute zincating baths in conjunction with the use of pyrophosphate copper. At present the only successful way to immersion zinc plate aluminum prior to pyrophosphate copper is to use conventional concentrated zincating baths which contain metallic salts as accelerators. The disadvantages of using concentrated zincates are explained in US. Patent 3,216,- 835.

Dilute zincate baths are those baths whose total salt content is preferably 180 g./l. or less. Such dilute zincate baths have the following advantages over conventional zincating formulations:

(a) The lower concentration results in much lower viscosities. This minimizes trapping of solution in porous die castings and the like. Such trapped solution is a source of blistering of subsequent electrodeposits.

ice

(b) The resulting lower viscosity leads to lessen dragout losses from operating tanks. This can be a sizeable economic factor where drag-out is high, as when the shape of the parts being plated in such as to trap substantial quantities of solution in crevices and irregularities of the surface under consideration.

(0) The more dilute the bath, the lower the initial cost of bath make-up tends to be.

((1) The more dilute zincate baths tend to operate faster than conventional baths. This is an advantage in the operation of fully automated plating installations. It is also advantageous when plating alloys which tend to form surface oxides rapidly.

' In the latter case it is necessary to form the zinc coating as rapidly aspossible.

Despite the above advantages, dilutezincate formulations have found relatively little commercial application compared to conventional, concentrated formulations. The reason is that the dilute zincate baths have been hard to control, with resulting tendency to formation of blisters after subsequent electrodesposits have been applied. In addition, any given dilute zincate bath has tended to work for only a limited number of alloys of aluminum. Thus, if a variety of alloys had to be processed, no one given formulation could be reliably utilized.

As pointed out in the aforesaid US. Patent 3,216,835, the principal shortcoming of previous dilute zincate baths is that the displacement deposit of zinc tends to form so rapidly as to be of unsound structure. Thus, such baths can be used only on some aluminum alloys which are suitable for such baths. Aluminum alloys containing magnesium and silicon, for example, cannot be treated reliably in such formulations. Previous attempts to remedy these defects have centered on the use of metallic activators, such as iron or copper or even lead, or on the use of complexing agents such as cyanide. Another approach has been to add small amounts of inorganic oxidizing agents, such as nitrite and/ or nitrate.

None of these attempts to overcome the objections to dilute zincate formulations have been fully successful. In particular, the above formulations often cause blistering after subsequent electroplating.

We have discovered a formulation and composition whereby the aforementioned defects may be overcome in a zincating operation wherein dilute zincate formulations containing the indicated complexing reagents may be employed and wherein such compositions and solutions containing them have the following characteristics:

(a) capable of chelating both aluminum and zinc; I

(b) capable of chelating strongly enough to prevent an unsound zinc deposit from forming, yet not so strongly as to slow down the deposition rate appreciably;

(c) capable of rapidly chelating aluminum at the interface between aluminum and the zincating solution.

Contrary to what might be expected by one skilled in the art, we have found that the presence of good complexing agents for aluminum and zinc, such as fluoride and cyanide, is actually deleterious. It is conceivable that this is related to the fact that the rate at which such complexes form, and dissociate, is relatively slow.

A further property of the formulations forming our invention is the ability to plate a sound zinc deposit amenable to subsequent copper strike on virtually all commercial aluminum alloys by suitable adjustment of (a) bath concentration, preferably in the range of 60480 g./l. total salt content, (b) temperature, preferably in the range of 45 -200 F., and (c) time of immersion, pref- :r-ably in the range of seconds to 2 minutes. The formulations which constitute our invention may be used in combination with our conventional treatment cycle for preparing the aluminum surface. By such conventional treatments, we understand use of etching or non-etching (silicated or otherwise) alkaline cleaners for aluminum; pickles, or etchants, such as those containing nitric acid; nitric-sulfuric acids; nitric-sulfuric-hydrofluoric acids (this being an illustrative, and not a restrictive listing); bright dips for aluminum, either acid or alkaline; use of either a single, double, or triple dip in the zincating bath, with intermediate removal of the immersion coating in any of the pickles or bright dips mentioned above.

We have now found that, in accordance with the present invention, baths of the character disclosed and claimed in aforementioned US. Patent 3,216,835, namely, a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound and a chelating reagent capable of chelating both aluminum and zinc including the combination of (1) from about 5 to 95 percent of a water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of a water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4, can be markedly improved by the addition of copper salts in an amount of from about A to about 1 /2 grams per liter copper ion content. The object to be plated is immersed in this particular solution, having an optimal or preferred concentration of grams per liter as copper. Because such baths deposit zinc immersion films quite rapidly, due to the well known accelerating effect of the copper ion, time of immersion should be rather short and temperatures generally kept at a minimal. Immersion times of from 3 to 30 seconds are suitable at temperatures of 65 to 150 F. and generally speaking the shorter immersion times are employed at the higher temperatures and the converse. However, the disclosure of the present invention is not necessarily restricted to these specified conditions.

The pretreatment of the aluminum surface also has an important bearing on the rate at which the immersion zinc deposit forms later. Without any intention to restirct the scope of the present disclosure, as different aluminum alloys and different operating conditions will at times indictae, we have found the following pretreatment is broadly useful:

(1) Surfaces should be etched in any commonly used proprietary etch-type aluminum cleaner, for example with a concentration of 30 grams per liter at a temperature of 70 C. for about a period of 5 seconds.

(2) The article is then thoroughly rinsed; and

(3) The article is subjected to an aqueous solution comprising approximately the following constituents:

Percent by volume Nitric acid (70%) 50 Sulfuric acid (98%) Approx. 25 Water Approx. 25

(4) The next step in the procedure is to rinse the article thoroughly.

(5) Then the article is zincated in accordance with the disclosed procedure in Patent 3,216,835 referred to above, modified as noted below.

The article thus treated is readily amenable to a pyrophosphate copper strike process whereupon the surface is rendered completely amenable to plating with a variety of metals.

4 SCOPE OF DISCLOSURE In the tables which follow, the data are taken from (a) Tables of Stability Constants (II Vols), J. Bjerrum, G. Schwartbenbach, L. D. Sillen, IUPAC (Chem. Soc., London, 1957); (b) The Sequestration of Metals, R. L. Smith (Macmillan, 1959); (c) The Chemistry of the Coordination Compounds, J. C. Bailar, Jr., Ed., A.C.S. (Reinhold, 1956) and (d) Chemistry of the Metal Chelate Compounds, A. E. Martell, and M. Calvin (Prentice- Hall, 1952).

The numbers given below (unless otherwise stated) are for the common logarithim of the stability constant, k defined as:

Where denotes activity, M is the metal ion involved and X is the chelate ion involved. In a few cases, other k values are indicated:

The chelating agents which we discovered to be effective (as additives to the basic dilute zincate formulations) may be grouped as follows:

(I) Amines A. Monoamines.

B. Diamines. C. Polyamines.

(II) Carboxylic acid or corresponding salt a-Hydroxy carboxylic.

. Di-carboxylic. 0cand fi-Keto carboxylic.

(III) Other oxy-compounds where hydrogen constitutes 0-2 of the R members, and the balance of the R members contain l-4 carbons in each R member and at least one of the R members contains a hydroxy or carboxylic grouping. Examples are given in the following Table IA.

the balance of the R members each contain 1-4 carbons at least one of the R members containing a hydroxyor TABLE IA Name Formula Log in Zn Al Aminoacetate (glycinate) NH:A(5O 5. 5 ma i 2-aminopropionate (alaninate) NHz(JH(CHa)-COO 5.2

Amino-di(hydroxyethyl) acetate (dihy- N-(CHah-OH 5. 4

droxyethyl glyeinate).

(CHM-0H Amino acetopropionate NH-PrO 6.2

Amino diacetate NH-Ac0 7. 0

Aminomethyl diacetate N-CH; 7.0

AcO

(CHmOH Amtnohydroxyethyl diacetate NAe0 8. 5

AcO

PrO Amino propionate diacetate N AeO 9. 8

A00 Aminotriacetate (nltrilotriacetate; NAc0 10.5 10

NTA). \A

where hydrogen constitutes 0-2 of the R members, and

carboxylic grouping; further wherein R contains either 2-3 carbons, with or without hydroxyl side chains, or

9 else R contains one or more ethyl ether groups. Ex-

amples are given in the following Table IB.

TABLE 113 Name Formula Log k1 Al OAe\ v Ethylenediamine'N 1N diacetate N-(CH2)2NH2 11.9

, 7. Efhylenediamine tetraacetate (EDTA) N(CH2)2-N\ 16.0 16.1

OPr AcO Ethylenediaminediaeetate dipropionate N(CH2)2-N 14. 5

' OPr AcO TABLE IB.-C0utlnued OPr PrO Ethylenediamine tetrapropionate N--(C Hi) ;-N 7. 8

OPr PrO HO-(CHz) A00 Ethlenediaugiue (hydroxyethyl) triaeetate N-(CHa) :N 14. 5

OAe AcO 0A0 A00 Propylene 1,2 diamine tetraacetate NCH CHCH; 16. 2

OAc CH; A00

0A0 AcO 1,3-dia-mino-2 propanol tetraacetate NC HiCH'-CH2N 12. 9 14. 4

OAc ()H AeO 0A0 A00 Diaminodlethyl-ether tetraacetate N- (0 HQ) ;0 C H;) 1-N 14. 9

0A0 AeO OAc A00 Diaminotrlethyl dlether tetraacetate N[(CH;);O];(CH7);N -14 a OAc AcO C. Polyamines: We also discovered as having similar chelating action as Classes IA and 1B, the use of poly: amines of the general formula R2 Rl where hydrogen constitutes 0-4 of the R members, the balance of the R members each containing 1-4 carbons,

at least one of the R members containing a hydroxyor 50 earboxylic grouping; further, wherein R is either hydrogen or an organic group as in the case of R members; further, wherein x is 1-4. Examples are given in the following I able 1C.

having a log k zinc stability constant of about 1.5 to 4, particularly a-hydroxy carboxylic acids (or their salts) of the general formula 'where hydrogen constitutes 02 of the R members, and

the balance of the R members contain l4 carbons in combination with chelating agents of classes IA, 1B, 1C and IIIA function to activate the zincating process. Such carbon-containing R members may be hydro carbons,

40 such as alkyl groups, but preferably contain oxygen, such (II) Carboxylic acid or corresponding salt.-A. a-Hyas carboxylic, aldehydic, or hydroxyl groups. Examples droxy carboxylic: We have found that these compounds 75 are given in the following Table IIA.

TABLE IIA Name Formula log k1, Zn Al Glycollate 000- 1.9

CHQOH Lactate 000-- 2.2 I (k1g=3.4)

OHOH

CH3 l0 Glycerate (ilOO- 1.8

(llHOH OHzOH Malate COO- 3.7

I OHOH IE2 CO0- Tartrate 000- 27 |H011 OHOH Gluconate 000- 1.7

(+ h CHrOH 3O Saccharate OOO- COO- Citrate COO- HO- COO" CH2 CO0- B. Dicarboxylic: These compounds having a similar log k zinc stability constant as Class IIA, namely, dicarboxylic acids (or their salts), of the general formula I coon u I I s u where R contains 0-2 carbons likewise function effectively in the combination. Examples are given in the following Table IIB.

TABLE IIB Name Formula log k1, Zn Al Oxalate 000- 4.7 (kn=7. 2) (k,2= 00- (ki2s=8- 12s Malonate 000- 3.2

COO

Succinate CIOO- 2.7

C. uand fl-Keto carboxylic: The use of u-keto carboxylic acids (or their salts) 'of the general formula wherein R contains 1-3 carbons in a chain, with or without side chains in the combination produces novel 10 and unexpected chclating results. While not necessary, it is preferable that carbons in the ,B-position to the CO group in the formula above contain hydroxy or carboxylic groupings. We further claim fl-keto carboxylic acids (or their salts) of the general formula wherein R is defined as above. Examples are given in the following table.

(III) Other oxy-compounds.-A. a-Hydroxy alcohols: These compounds having a similar log k zinc stability constant as Classes HA and IIB, namely, the use of ahydoxy alcohols of the general formula CHQOH wherein R is preferably hydrogen, but may also represent a chain containing l-3 carbons; wherein R may be hydrogen, or may represent a chain of 1-4 carbons, said carbons preferably, but not necessarily, containing hydroxyl groupings are likewise effective in the combination. Examples are given in the following table.

TABLE IIIA Name Formula Log k1 Zn CHrOH C ZH2OH OHiOH JlIOH 6H3 CHzOH CHOH mon CHzOH (CHOH)4 CHgOH Ethylene glycol Propylene glycol Glycerol Mannitol; Sorbitol B. p-Diketonesz These compounds, namely, fi-diketones of the general formula Where R and R have 1-3 carbons, with or without side chains have a similar chelating action in the combination as classes IIA, IIB and HG. An example is given in the following table.

TABLE IIIB Name Formula Log k1 Zn Al Acetylacetone CHs-C O-CHz-C O-OH; 5. 1 8.6

( 12=9. (km =16. m

C. a-Hydroxy aryl compounds: These compounds, namely, the use of oc-hYdI'OXY aryl compounds of the general formula lyzed grouping, such as carboxyl grouping. Examples are given in the following table.

TABLE 1110 Name Formula Log In Zn Al Cateehol -O H Salieylate 0 H 14 Sull'osalieylate 0 H Good Acetylsalieylate O O O C H; 14

Salioylaldehyde O H 4. 5

Sulfosaliey1aldehyde 0 H 3. O

-OaS CHO 'D. Alizarin Derivatives: Such compounds namely, the use of alizarin and its derivatives have similar chelating action in the combination as class IIIC. Examples are given in the following table.

TABLE IIID Name Formula Log k1 Zn Al Alizarin [I l O OH Alizarin sulfonate S 0;:

l I 0 O H An examination of the tables will show that relatively little data are available for aluminum, but somewhat more for zinc. It will be noted that the stability constants for zinc chelates fall into two broad categories:

(a) log k, is 4.5-18 in Tables I and III B-D (b) log k is 1.54 in Tables 11 and IIIA Qualitative data in the literature indicate a similar classification for aluminum. There are no quantitative data for Table IIIA, but qualitative data from the literature suggest the classification given above.

On the basis of the classification given above, it might at first seem that the chelating agents of Tables I and III B-D would be superior to those of Tables II and IIIA. However, we have discovered a synergistic relationship between the two above classifications of chelating agents which would not be evident to one skilled in the art of either chelating agents in deposition solutions or of plating on aluminum.

Accordingly, in addition to the agents referred to previously, and subject to limitations imposed elsewhere, the use of any of the following classes of chelating agents: IA, IB, IC, IIIB, IIIC, IIlD (as heretofore defined), singly or in combination, in connection with the use of any of the following classes of chelating agents: IIA, IIB, IIC, IIIA (as heretofore defined), singly or in combination. We have discovered the surprising and unexpected result that zincating formulations containing the two above classes of chelating agents conjunctively are more efiicacious than those employing chelating agents drawn from only one of the above two classifications.

We believe that the synergistic action of a combination of chelating ingredients producing the desired zincating action is unpredictable and would hypothesize this action, without restriction as to the scope of the specification and claims as follows:

Zincating of aluminum surfaces is a galvanic displacement reaction, wherein zinc ions are reduced to zinc metal, and aluminum is oxidized to aluminum ions. If the free ions were employed (as in acid solutions, for example), the resulting galvanic deposits would be dendritic (i.e., treed, of coarse, and non-coherent crystalline structure). Thus it is necessary to complex or chelate the ions involved. The complexing agent most commonly employed is hydroxide, i.e., highly alkaline solutions are employed. The stability constants for hydroxide complexes are: log k 14.9 (Zn); 33.8 (Al). Thus, in theory, the so called zincating of aluminum surfaces would take place by the following oxidationreduction reaction:

13 In practice, however, the rate at which such hydroxide complexes form is slow. Thus, if the zincating reaction is too fast, aluminum hydroxide will precipitate on the surface and be incorporated with the zinc deposite, leading to unsatisfactory results. A simple zincating formulation, i.e., NaOH a zinc salt, is satisfactory for conventional concentrated zincating baths. However, the dilute baths operate more rapidly, so rapidly indeed, that hydroxide precipitates form on the surface. Thus, in dilute zincating formulations, it is necessary to employ chelating agents which will rapidly chelate aluminum (to prevent precipitate formation), and which will rapidly liberate zinc ions (to permit the oxidation-reduction reaction to proceed in the first place). It would appear (based on practical results obtained, but not on kinetic or other 14 in U.S. Patent 3,216,835, are modified by the addition of from about 0.4 part to 0.85 part of copper ion supplied typically by the addition of 1.6 to 3.4 parts of copper sulfate (CuSO .5H O). The sodium hydroxide content is reduced accordingly. As noted in the earlier disclosure of Patent 3,216,835, the salt content is approximately from 60 to 180 grams per liter and accordingly 0.4% copper ion at 60 grams per liter is approximately A grams per liter copperat a minimum and about 0.85% copper ion at 180 grams per liter is equivalent to 1 /2 grams per liter copper at a maximum. By use of 0.625% copper (for example 2.5% copper sulfate), the preferred amount of 4 grams per liter of copper ion is achieved. Exemplary of compositions falling within the scope of the present disclosure can be seen from the following table.

Parts (wt.)

O Acetyl salicylate (Na) Amino Triacetate (Na).

Catechol.

Diethylene triamine peutaacetate (Na) Ethylenediamine tetraacetate (Na) Gluconate (Na) Rochelle Salt. 6. 7 Wetting Agent (anionic) 1.0 Copper sulfate pentahydrate 1. 6

rate-indicative data) that chelates of groups II and IIIA permit such rapid chelating action. However, their use alone is not the optimum solution of the problem, since the stability constants are low, thus leading to the eventual precipitation of hydroxides somewhere in the general vicinity of the aluminum surface. Convection and diffusion in the solution will then tend to re-deposit such hydroxides on the surface being plated. By also adding chelates of types I and III BD, this problem is overcome since these have much higher stability constants. However, when chelates of the latter types are used alone, less than optimum results are obtained since their rate of chelate formation with Al, and liberation of Zn ions is slower (based on practical observations, and not on kinetic data). Thus, there is some risk of hydroxide precipitate formation directly on the surface of the work. While hypothetical, the above model would account for the unexpected synergistic effects noted when both classes of chelating agents are present.

The chelating agents of the present invention are preferably used in formulations containing about 60-180 g./l. of total salts. Of the total salt content, the following are present in the amounts indicated:

(a) An .alkali metal hydroxide, preferably sodium h y droxide, about 60 to 85 parts by weight.

(b) A zinc salt (such as zinc oxide, zinc sulfate, etc.) about 7 to 15 parts by weight based on the zinc content. A preferred economical salt is zinc oxide.

(c) Weight ratio of OH /Zn (as metal): about 2.1- 7.9 (Expressed as NaOH/ZnO: (4-15).

((1) A chelating reagent in .an amount of from about 5 to parts by weight comprising the combination of (1) from about 5 to 95 percent of at least one water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of at least one water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4.

(3) Optionally, surface active agents may be present to lower surface tension up to about 2 parts by weight.

These formulations, which are disclosed and claimed Although we have referred herein to the specific copper salt as copper sulfate pentahydrate, it is to be understood that any appropriate ionizable copper salt is appropriate which requires the proper amount of copper ion. That is, copper nitrate would provide the necessary co-pper ion to create the required conditions.

Treated in accordance with the foregoing procedure, the zincated aluminum or aluminum .alloy is appropriately prepared for the pyrophosphate copper strike. An example of the novel art of pyrophosphate copper strike is found in the Second Edition of Modern Electroplating (Wiley, 1963) pp. 202-205. Herein is quoted the use of copper pyrophosphate plating baths containing:

For more operable copper strikes, it is suggested that the above be diluted.'No limitation on ranges are included but dilutions of up to 1 to 3 would be typical,

What we claim is:

1. A zincating bath for aluminum metals comprising a dilute aqueous solution of a Zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound, a chelating agent capable of chelating both aluminum and zinc comprising the combination of (1) from about 5 to percent of a water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of a water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and a small amount sufficient to passivate the zinc immersion deposit sufliciently to prevent an immersion copper deposit from forming thereon in a subsequent pyrophosphate copper strike solution, of a copper salt.

2. A zincating bath for aluminum metals comprising a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound, a chelating agent capable of chelating both aluminum and zinc comprising the combination of (1) from about to 95 percent of .a water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from'about 95 to 5 percent of a water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and a copper salt in an amount of about A to 1 /2 grams per liter of copper ion content.

3. A zincating bath for aluminum metals comprising a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound, a chelating agent cap-able of chelating both aluminum and zinc comprising the combination of (1) from about 5 to 95 percent of a water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of a water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and a copper salt concentration of about gram per liter.

4. A process for the preparation of aluminum and aluminum alloys for plating with pyrophosphate copper comprising the steps of treating the surface of the aluminum with a solution comprising a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound and a chelating reagent having a log k zinc stability constant of about 4.5 to 18 and a water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and a small amount suflicient to passivate the zinc immersion deposit sufliciently to prevent an immersion copper deposit from forming thereon in a subsequent pyrophosphate copper strike solution, of a copper salt.

5. A process for the preparation of aluminum and aluminum alloys for plating with pyrophosphate copper comprising the steps of treating the surface of the aluminum with a solution comprising a dilute aqueous solution of a zinc salt having a log k zinc stability constant of about 4.5 to 18 and a Water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and a copper salt having a concentration of about A to 1 /2 grams per liter based on copper prior to subjecting the article to pyrophosphate copper plating.

6. A process for the preparation of aluminum and aluminum alloys for plating with pyro-phosphate copper comprising the steps of treating the surfaceof the aluminum with a solution comprising a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about 180 grams per liter, an alkaline compound and a chelating reagent having a log k zinc stability constant of about 4.5 to 18 and a water soluble chelating agent having a log k zinc stability constantof about 1.5 to 4, said solution containing a copper salt at a concentration of approximately gram per liter as copper.

7. A process as set forth in claim 6, wherein the article is immersed at a period of time of 3 to 30 seconds.

8. A process as set forth in claim 6, wherein the article is subjected to a temperature of about 65 to 150 F.

9. A process as set forth in claim 6, wherein the surface is etched with an ordinary aluminum etched type cleaner having a concentration of 30 grams per liter at a temperature of about 70 C. for a period of about 5 seconds.

10. A process as set forth in claim 6, wherein the article to be plated is subjected to an aqueous mixture of nitric acid and sulfuric acid and thereafter rinsed prior to zincating.

11. A zincating composition for preparing aluminum metal surfaces for plating comprising about 60 to 85 parts by Weight of an alkali metal hydroxide, about 7 to 15 parts by weight of a zinc salt and about 5 to 20 parts by weight of a chelating reagent capable of chelating both zinc and aluminum comprising, in combination, (1) from about 5 to 95 percent of a chelating agent having a log k Zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of a chelating agent having a log k zinc stability constant of 1.5 to 4 and from about 0.4 to 0.85 .parts of copper ion.

12. A zincating composition for preparing aluminum metal surfaces for plating comprising about 60 to parts by weight of an alkali metal hydroxide, about 7 to 15 parts by weight of a zinc salt'and about 5 to 20 parts by weight of a chelating reagent capable of chelating both zinc and aluminum comprising, in combination, (1) from about 5 to percent of a chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of a chelating agent having a log k zinc stability constant of 1.5 to 4, the weight ratio of hydroxyl ions to zinc being about 2.1 to 7.9 and from about 0.4 to 0.85 part of copper ion.

13. A zincating composition for preparing aluminum metal surfaces for plating including in combination a zinc salt, an alkali and (1) from about 5 to 95 percent of at least one water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of at least one water soluble chelating agent having a log k zinc stability constant of about 1.5 to 4 and from about 0.4 to 0.85 parts of copper ion.

14. A zincating composition for preparing aluminum metal surfaces for plating including in combination a znc salt, an alkali and (1) from about 5 to 95 percent of at least one water soluble chelating agent having a log k zinc stability constant of about 4.5 to 18 selected from the class consisting of the monoamines, polyamines and their salts p-diketones, u-hydroxyaryl compounds and alizarin derivatives and their salts, and (2) from about 95 to 5 percent of a chelating agent having a log k zinc stability constant of about 1.5 to 4 selected from the class consisting of oc-hYdlOXY carboxylic acids and their salts, dicarboxylic acids and their salts, ocand B-keto carboxylic acids and their salts and a-hydroxy alcohols and from about 0.4 to 0.85 parts of copper ion.

15. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an amine and an a-hydroxy carboxylic acid and from about 0.4 to 0.85 part of copper ion.

16. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an amine and a dicarboxylic acid, and from about 0.4 to 0.85 part of copper ion.

17. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an amine and an ot-keto carboxylic acid and from about 0.4 to 0.85 part of copper ion.

18. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an amine and a fi-keto carboxylic acid and from about 0.4 to 0.85 part of copper ion.

19. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an amine and an a-hydroxy alcohol and from about 0.4 to 0.85 part of copper ion.

20. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of a [i-diketone and an a-hydroxy carboxylic acid and from about 0.4 to 0.85 parts of copper ion.

21. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of a fl-diketone and a dicarboxylic acid and from about 0.4 to 0.85 parts of copper ion.

22. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of a B-diketone and an a-ketone carboxylic acid and from about 0.4 to 0.85 parts of copper ion.

23. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of a [i diketone and a fi-keto earboxylic acid and from about 0.4 to 0.85 parts of copper ion.

24. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of a ,6- diketone and an a-hydroxy alcohol and from about 0.4 to 0.85 parts of copper ion.

25. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an hydroxyaryl compound and an Ot-hYdIOXy carboxylic acid and from about 0.4 to 0.85 parts of copper ion.

26. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an achydroxyaryl compound and a :dicarboxylic acid, and from about 0.4 to 0.85 parts of copper ion.

27. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an ochydroxyaryl compound and an a-keto carboxylic acid, and from about 0.4 to 0.85 parts of copper ion.

28. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an ahydroxyaryl compound and a pI-keto carboxylic acid, and from about 0.4 to 0.85 part of copper ion.

29. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an a- 18 hydroxyaryl compound and an u-hydroxy alcohol and from about 0.4 to 0.85 part of copper ion.

30. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an alizarin derivative and an a-hydroxy earboxylic acid and from about 0.4 to 0.85 part of copper ion.

31. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an alizarin derivative and a dicarboxylic acid, and from about 0.4 to 0.85 part of copper ion.

32. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an alizarin derivative and an oc-kCtO carboxylic acid and from about 0.4 to 0.85 part of copper ion.

33. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an alizarin derivative and a p-ket-o carboxylic acid and from about 0.4 to 0.85 part of copper ion.

34. A zincating composition as set forth in claim 14, wherein the chelating reagent is the combination of an alizarin derivative and an a-hydroxy alcohol and from about 0.4 to 0.85 part of copper ion.

No references cited.

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner. 

1. A ZINCATING BATH FOR ALUMINUM METALS COMPRISING A DILUTE AQUEOUS SOLUTION OF A ZINC SALT HAVING A TOTAL SALT CONCENTRATION OF NO MORE THAN ABOUT 180 GRAMS PER LITER, AN ALKALINE COMPOUND, A CHELATING AGENT CAPABLE OF CHELATING BOTH ALUMINUM AND ZINC COMPRISING THE COMBINATION OF (1) FROM ABOUT 5 TO 95 PERCENT OF A WATER SOLUBLE CHELATING AGENT HAVING A LOG K1 ZINC STABILITY CONSTANT OF ABOUT 4.5 TO 18 AND (2) FROM ABOUT 95 TO 5 PERCENT OF A WATER SOLUBLE CHELATING AGENT HAVING A LOG K1 ZINC STABILTIY CONSTANT OF ABOUT 1.5 TO 4 AND A SMALL AMOUNT SUFFICIENT TO PASSIVATE THE ZINC IMMERSION DEPOSIT SUFFICIENTLY TO PREVENT AN IMMERSION COPPER DEPOSIT FROM FORMING THEREON IN A SUBSEQUENT PYROPHOSPHATE COPPER STRIKE SOLUTION, OF A COPPER SALT. 